The basic idea of external heat engines of the reciprocating piston type using an air heater in which the compressed air is heated from the outside and in which the heating gases never mix with the compressed air within the heater has been known for quite a long time. The first known U.S. patent relating to this kind of engine is U.S. Pat. No. 1,038,805, granted to S. J. Webb in 1912. That engine lacked many important feature which would be needed to make it practical. Only two other U.S. patents are known to have been issued on this kind of engine, both of which are of the closed cycle type and therefore cumbersome, expensive to manufacture and not practical for general use. One basic feature which all three engines lack is a space for the heated air to expand under substantially constant pressure and to be accommodated before being admitted into the power cylinders. No engine of this kind will perform adequately, if it will run at all, without such expansion space, as hereinater explained.
The lack of the above mentioned feature and other shortcomings preventing this kind of engine from being successfully employed in the economy of this country and the world have been taken into consideration in this invention and shall be explained hereinafter.